(1) Field of the Invention
The present invention relates to a polarized light synthesizing apparatus, a polarized light splitting apparatus and a pump light outputting apparatus suitable for use in an optical amplifier for amplifying signal light in an optical communication system.
(2) Description of the Invention
There have been developed optical amplifiers each of which can amplify an optical signal without converting the optical signal into an electric signal in these years with development of an optical communication system. Among them, an optical fiber amplifier using an optical fiber which is doped with an rare-earth element such as erbium (Er) or the like (EDF: Erbium-Doped Fiber) has features such as high gain, low noise and the like, playing an important role in an optical communication system.
Meanwhile, there have been employed WDM (Wavelength Division Multiplexing) optical communication system, TDM (Time Division Multiplexing) optical communication system and the like, as systems for transmitting and communicating signal light of a plurality of channels over one optical fiber simultaneously.
An optical fiber amplifier (hereinafter, called an optical amplifier) used in the WDM optical communication system amplifies input signal waves (signal light) of a plurality of channels such as 4, 8, 16, 32 or 64 waves to output light of high power. Therefore, the optical amplifier requires a high power pumping light source since a pumping power required to amplify the waves is increased as signal light to be amplified is increased.
In order to obtain a high power output of the pumping source, it is necessary to consider employment of techniques for a high power pumping laser, bi-directional pumping, polarization synthesis of pump light, wavelength synthesis of pump light, and the like.
However, in order to realize the technique such as, in particular, polarization synthesis, wavelength synthesis or the like, an optical circuit device adaptable thereto becomes necessary. If devices for respective functions are configured, problem such as an increase in size of the apparatus, an increase of loss caused by insertion of the optical device and the like will arise. It is therefore desirable to integrate the function of polarization synthesis and the function of wavelength synthesis of pump light to configure an optical circuit device (pump light outputting apparatus), thereby suppressing an increase in size of the apparatus or an increase of insertion loss.
FIG. 11 is a plan view schematically showing a structure of a known pump light outputting apparatus. The pump light outputting apparatus shown in FIG. 11 multiplexes pump light obtained by polarization-synthesizing and wavelength-synthesizing four kinds of light and signal light amplified by an EDF. In the pump light outputting apparatus, there are integrated and realized not only the function of polarization-synthesizing pump light and the function of wavelength-synthesizing the same but also a function of synthesizing signal light to be amplified and outputting the same.
The pump light outputting apparatus shown in FIG. 11 has a first polarized light synthesizing unit 51, a second polarized light synthesizing unit 52, wavelength-division multiplexing films (WDMs) 504 and 505, an isolator 506, signal light collimators 510e and 520 and a casing 501.
The first polarized light synthesizing unit 51 has polarized light synthesizing collimators 510a and 510b, a high reflection mirror (HR) 502 and a polarization beam splitter (PBS) 503.
The polarized light synthesizing collimator 510a is configured with a polarization maintaining optical fiber 511a and a collimator lens 512a secured thereto. Similarly, the polarized light synthesizing collimator 510b is configured with a polarization maintaining optical fiber 511b and a collimator lens 512b secured thereto.
These polarized light synthesizing collimators 510a and 510b are secured on an outer periphery of the casing 501. On the other hand, the collimator lenses 512a and 512b are secured inside the casing 501. Pump light inputted from the polarization maintaining optical fibers 511a and 511b is emitted toward the inside of the casing through the collimator lenses 512a and 512b.
The polarization maintaining optical fibers 511a and 511b are configured with birefringent optical fibers, each of which can transmit light at a wavelength of, for example, about 1.46 .mu.m from a pumping source not shown as linearly polarized light (light whose electric field vector always points toward a constant direction) while maintaining a plane of polarization. The linearly polarized light that the polarization maintaining optical fibers 511a and 51b can propagate has electric field vectors pointing different directions. For example, the polarization maintaining optical fiber 511a propagates S wave, whereas the polarization maintaining optical fiber 511b propagate P wave.
The HR 502 totally reflects incident light, which is secured such as to emit pump light of S wave at a wavelength of 1.46 .mu.m inputted from the polarized light synthesizing collimator 510a toward the PBS 503.
The PBS 503 has a property that a transmittance thereof is varied according to a plane of polarization (direction of vibration) of incident light, which totally transmits light of P wave (at a wavelength of 1.46 .mu.m) emitted from the polarized light synthesizing collimator 510b, while totally reflecting light of S wave (at a wavelength of 1.46 .mu.m) emitted from the polarized light synthesizing collimator 510a through the collimator lens 512a. The PBS 503 is such secured as to multiplex (polarization-synthesize) the pump light of S wave from the above polarized light synthesizing collimator 510a and the pump light of P wave from the polarized light synthesizing collimator 510b into light of the same optical axis, and emits the light to the WDM 504.
The second polarized light synthesizing unit 52 has a polarized light synthesizing collimators 510c and 510d, a HR 507 and a PBS 508, similarly to the first polarized light synthesizing unit 51. The polarized light collimators 510c and 510d are secured to the outer periphery of the casing 501. On the other hand, the collimator lenses 512c and 512d are secured inside the casing 501. Pump light inputted from the polarization maintaining optical fibers 511c and 511d is emitted to the inside of the casing 501 through the collimator lenses 512c and 512d.
The polarization maintaining optical fibers 511c and 511d are configured with birefringent optical fibers, each of which can transmit light at a wavelength of about 1.48 .mu.m, for example, from a pumping light source not shown as linearly polarized light while maintaining a plane of polarization. The linearly polarized light that the polarization maintaining optical fibers 511c and 511d can propagate has electric field vectors pointing different directions. For example, the polarization maintaining optical fiber 511c propagates S wave, whereas the polarization maintaining optical fiber 511d propagates P wave.
The HR 507 totally reflects incident light, similarly to the HR 502, which is secured so as to emit the pump light of S wave at a wavelength of 1.48 .mu.m inputted from the polarized light collimator 510c toward the PBS 508.
The PBS 508 has a property that a transmittance thereof is varied according to a plane of polarization (direction of vibration) of incident light, similarly to the PBS 503. The PBS 508 totally transmits light of P wave (at a wavelength of 1.48 .mu.m) from the polarized light synthesizing collimator 510d, while totally reflecting light of S wave (at a wavelength of 1.48 .mu.m) from the polarized light synthesizing collimator 510c.
The PBS 508 is secured so as to multiplex (polarization-synthesize) the pump light of S wave from the above polarized light synthesizing collimator 510c and the pump light of P wave from the polarized light synthesizing collimator 510d into the same optical axis. The polarization-synthesized pump light passes through the WDM 504 and the WDM 505, then is emitted to the signal light collimator 520.
The WDM 504 and 505 are chemical films each having a property that a transmittance thereof is varied according to a wavelength of incident light. The WDM 504 totally transmits light at a wavelength of 1.48 .mu.m, while totally reflecting light at a wavelength of 1.46 .mu.m. On the other hand, the WDM 505 totally transmits incident light at wavelengths of 1.48 .mu.m and 1.46 .mu.m from the WDM 504, while totally reflecting incident signal light (at a wavelength of, for example, about 1.55 .mu.m) from the signal light collimator 510e.
Namely, the WDM 504 is disposed at such an angle as to direct a polarization-synthesized light at a wavelength of 1.46 .mu.m emitted from the first polarized light synthesizing unit (namely, the PBS 503) to an optical axis of polarization synthesized light at a wavelength of 1.48 .mu.m emitted from the second polarized light synthesizing unit 52 (namely, the PBS 508) toward the signal light collimator 520 and emit the light toward the signal light collimator 520. The WDM 504 totally transmits the incident polarization synthesized light at a wavelength of 1.48 .mu.m from the PBS 508, while totally reflecting the incident polarization-synthesized light at a wavelength of 1.46 .mu.m from the PBS 503, thereby wavelength-division-multiplexing the pump light so that the pump light has the same optical axis, and emits the pump light to the signal light collimator 520 through the WDM 505.
The WDM 505 multiplexes (wavelength-division multiplexes) the multiplexed light configured with the pump light (polarization-synthesized light) at a wavelength of 1.48 .mu.m and the pump light (polarization-synthesized light) at a wavelength of 1.46 .mu.m, and signal light (at a wavelength of 1.55 .mu.m) emitted from the signal light collimator 510e, and emits the multiplexed signal light (including the pump light components) toward the signal light collimator 520.
The signal light collimator 520 has, similarly to the polarized light synthesizing collimators 510a through 510d, has an optical fiber 521 and a collimator lens 522. The signal light collimator 520 is disposed on the outer periphery of the casing 510, and the collimator lens 522 is disposed inside the casing 501, whereby the signal light passing through the collimator lens 522 is emitted to the optical fiber 521.
The signal light collimator 510e has, similarly to the polarized light synthesizing collimators 510a through 510d and the signal light collimator 520, an optical fiber 511e and a collimator lens 512e. The signal collimator 510e is disposed on the outer periphery of the casing 510, whereas the collimator lens 512 is disposed inside the casing 510, whereby the signal light emitted from the optical fiber 511e is converted into a collimated beam by passing through the collimator lens 512e, and emitted toward the WDM 505.
The isolator 506 is disposed on an optical axis of the signal light emitted from the signal light collimator 510e toward the WDM 505 to prevent resonance of the apparatus due to reflection of the signal light.
In the above structure, signal light is inputted to the optical fiber 511e of the signal light collimator 510e, and simultaneously pump light is emitted from the pumping lasers disposed to the polarized light synthesizing collimators 510a through 510d to the polarization maintaining optical fibers 511a through 511d. The first polarized light synthesizing unit 51 polarization-synthesizes pump light of S wave and P wave at a wavelength of 1.46 .mu.m, whereas the second polarized light synthesizing unit 52 polarization-synthesizes pump light of P wave and S wave at a wavelength of 1.48 .mu.m, further the WDM 504 wavelength-division-multiplexes the pump light. Signal light emitted from the signal light collimator 510e is wavelength-division-multiplexed over the wavelength-division-multiplexed pump light, then outputted from the signal light collimator 520 to an EDF not shown.
As the polarized light synthesizing unit for polarization-synthesizing pump light, there is known a polarized light synthesizer disclosed in Japanese Patent Laid-Open Publication No. 6-148570. The known polarized light synthesizer disclosed in the Japanese Patent Laid-Open Publication No. 6-148570 will be hereinafter described with reference to FIG. 12. FIG. 12 is a diagram schematically showing a structure of the polarized light synthesizer.
The polarized light synthesizer has, as shown in FIG. 12, birefringent crystals 3 and 6, a focusing rod lens 41, a high reflection mirror 5, input optical fibers 1 and 2, and a transmission fiber 7, which synthesizes linearly polarized light inputted through the input optical fibers 1 and 2, and outputs synthesized light from the transmission fiber 7.
The birefringent crystals 3 and 6 are uniaxial birefringent crystals such as rutile (TiO.sub.2), crystal or the like. Inside the birefringent crystals 3 and 6, light porlarized in a direction of the optical axis advances straight as an ordinary ray, whereas light polarized in a direction perpendicular to the optical axis changes its path with an angle and advances as an extroadinary ray.
The input optical fibers 1 and 2 are connected to an end of the birefringent crystal 3. The input optical fibers 1 and 2 are polarization maintaining fibers each of which propagates light from a semiconductor laser (not shown) in a state where the light maintains linear polarization. At the other end of the birefringent crystal 3, there is connected an end of the focusing rod lens 41. The high reflection mirror 5 is adhered to the other end of the focusing rod lens 41. There is also connected the birefringent crystal 6 along with the birefringent crystal 3 to the other end of the focusing rod lens 41. The transmission fiber 7 is connected to the other end of the birefringent crystal 6.
In the above structure, when linearly polarized light from the semiconductor lasers not shown is inputted to the birefringent crystal 3 through the input optical fibers 1 and 2, the incident light from the input optical fibers 1 and 2 is separated into an ordinary ray and an extraordinary ray, and emitted to an end of the focusing rod lens 41.
The incident light is converted into parallel light by the focusing rod lens 41, after that, is reflected on a lens optical axis 8 of the high reflection mirror 5, again passes through the focusing rod lens 41, then is inputted to the birefringent crystal 6.
Inside the birefringent crystal 6, the incident light is again separated into an ordinary ray and an extraordinary ray and advances inside the crystal 6. Although the light separated into the ordinary ray and the extraordinary ray has advanced in the birefringent crystal 3, this time the ordinary ray is changed to the extroadinary ray and the extraordinary ray changed to the ordinary ray and advance in the birefringent crystal 6, and these rays are outputted in coincidence with each other from the other end of the biregrigent crystal 6 (the surface connected to the transmission fiber 7), then synthesized and coupled in the transmission fiber 7.
However, the pump light outputting apparatus shown in FIG. 11 requires polarization ports for P wave and S wave since the pump light outputting apparatus polarization-syntheses P wave and S wave at one wavelength. The device integrated polarization-synthesis and wavelength-synthesis therein requires a P polarization port for a short wavelength, an S polarization port for a short wavelength, a P polarization port for a long wavelength, an S polarization port for a long wavelength and an output port for outputting synthesized pump light, that is, 5 ports in total. The device is in a packaging size of L=about 90 mm and W=about 60 mm, for example. Therefore, it is impossible to realize a large decrease in packaging size only by integrating the functions.
In the polarized light synthesizer shown in FIG. 12, it is necessary to dispose the birefringent crystals 3 and 6 in a very small space between the input optical fibers 1 and 2 and the transmission fiber 7, and the focusing rod lens 41, so that a size of the birefringent crystal 3 and 6 has to be small and highly accurate dimensions are required in processing and assembling the device, which leads to a low productivity and a high manufacturing cost.
In assembling and fabricating the polarized light synthesizer shown in FIG. 12, the birefringent crystals 3 and 6, and the transmission fiber 7 are chemically adhered by adhesive not shown. However, the adhesive has a property of an inferior resistance to light since the adhesive is organic. In a state where light of a high power concentrates on a point within a very small area such as a core area of an optical fiber in, for example, an optical amplifier, the adhesive is easily damaged, thus the reliability is decreased. This causes harm when the apparatus is improved to have a high power, which is a fatal defect of the optical amplifier used essentially in an assumption that the optical amplifier operates with a high power (for example, light of high power such as 400 W concentrates on an area in diameter of about 10 .mu.m when four types of light each of 100 mW are multiplexed).